If you are the type that trains with Pokemon that are of a bit of a weaker level, try Route 14. If you want to train with Pokemon of around the same level, try Route 15. If you are a bit more daring (or you're skilled enough) to try your hand at either the Frost Cavern or Route 16. If you're going to the Frost Cavern, I would recommend going all the way to the end of the Frost Cavern by the Abomasnow. There is a healer nearby (breeder lady by the hiker south of the Abomasnow's room) and it's an easy place to run around until you find some decent-leveled Pokemon. I'm training a team right now, too, and I would recommend going in chronological order of all the routes at about 5 or 6-level intervals once your Gyarados gets too strong for the Pokemon in those areas. Hope this helps!